Cancers invoke molecular programs expressed during development and wound responses to promote the initiation and maintenance of complex neoplastic tissue systems that include not only transformed cells but also supportive vasculature, immune components, stroma, and extracellular matrix. Standard human cancer models based on established cell lines are subjected to passage under conditions that select for rapid proliferation and survival through mechanisms often distinct from the original tumor. While proliferation and resistance to apoptosis are hallmarks of cancer, other aspects of cancer—invasion of normal tissues, metastasis, resistance to cytotoxic insults and vascular recruitment—critically contribute to the lethality of cancer. An increasing number of cancers have been shown to display cellular hierarchies with a subset of the neoplastic compartment activating molecular mechanisms and cellular phenotypes similar—but not identical—to embryonic or tissue-specific stem cells. These cancer stem cells, also called tumor initiating cells (TICs) or tumor propagating cells, are functionally defined through assays of self-renewal and tumor propagation. It has been shown that TICs are relatively resistant to conventional cancer therapies (radiotherapy and chemotherapy) and promote tumor growth through angiogenesis. Bao et al., Cancer research, 66:7843-8 (2006); Cheng et al., Biochemical and biophysical research communications, 406:643-8 (2011). The TIC hypothesis has been questioned because of potential plasticity of the cellular hierarchy and difficulties with TIC identification but these challenges are products of the inventors' attempts to simplify complex systems with limited technical resources. Based on this background, creating agents that prospectively identify TICs may not only permit the interrogation of the cellular hierarchy in cancers but also serve as a platform for the development of novel targeted therapies and imaging reagents.
Glioblastoma (GBM) is the most prevalent and lethal primary brain tumor and ranks among the most lethal of all cancers. Through the work of many groups the presence of a cellular hierarchy has been supported in not only GBM but also other central nervous system cancers. Singh et al., Oncogene, 23:7267-73 (2004); Hemmati et al., PNAS, 100:15178-83 (2003). The characterization of TICs is based on paradigms developed from embryonic and tissue-specific stem cells but TICs are distinct from these normal cells so the immunophenotypes are likely non-overlapping in part. Several surface TIC markers have shown promise in GBM, including CD133 (Prominin-1) (Salmaggi et al., Glia, 54:850-60 (2006)), CD15/Lewis X-antigen/stage specific embryonic antigen-1 (SSEA-1) (Son et al., Cell stem cell, 4:440-52 (2009)), CD44 (Anido et al., Cancer cell., 18:655-68 (2010)), L1CAM (Bao et al., Cancer research, 68:6043-8 (2008)), integrin α6 (Lathia et al., Cell stem cell, 6:421-32 (2010)), epidermal growth factor receptor (EGFR) (Jin et al., Cancer research, 71:7125-34 (2011)), and platelet derived growth factor receptor β (PDGFRβ) (Kim et al., Genes & development, 26:1247-62 (2012)). Functional assays, including Aldefluor and side population, have been less reliable in GBM but useful in other cancer types. Broadley et al., Stem cells, 29:452-61 (2011).
Neurosphere formation has been used to enrich for TICs but this method prevents the prospective separation of tumorigenic and non-tumorigenic cells that define a cellular hierarchy. While these markers have been useful in some studies to prospectively enrich or deplete TICs, many of these have been limited by a shared antigen with normal neural progenitors. Several groups have taken other approaches to identify TIC targets. RNA interference screens have identified key transcription factors, kinases, phosphatases, or ubiquitin-modifying enzymes. Goidts et al., Oncogene, 31:3235-43 (2012). One group reported a TIC screen in prostate cancer stem cells (Sefah et al., Int. J. cancer, 132(11):2578-88 (2012)), and other identified CD133 binding aptamers (Shigdar et al., Cancer letters, 330(1):84-95 (2013)). However, a large screen to identify novel TIC enrichment reagents in GBM has not previously been developed.